Here’s a very thought-provoking article by Brian O’Sullivan on how elements of Irish culture and mythology have been used—and more often abused—in fantasy fiction. His main point is found, I think, in the these two paragraphs:
The problem, however, is that mythology is CULTURALLY based. Mythology contains elements of fantasy but at its most fundamental it’s an intellectual framework used by our ancestors to make sense of the world around them. Because it’s culturally based, many of the mythological elements and associated context have been passed down through generations and incorporated into national identity and belief systems. Today of course, the use of Irish mythology has been superseded by scientific rationale, but its core narratives remain intrinsically linked to Ireland’s self-identity and cultural values.
From an Irish perspective therefore, when you see your native cultural icons plucked from their normal environment, repackaged in some pseudo-Celtic [nonsense] and then reproduced out of context in a fantasy product, you can start to appreciate why other native groups complain about the commercial appropriation and exploitation of their cultures. For Irish people in particular, it feels as though we’ve been bombarded by mawkish, overly romanticised and culturally inaccurate interpretations of our own mythology for decades.
O’Sullivan is clearly and rightly passionate about this, and he provides much food for thought. I’m grateful for his raising my awareness of how things things are experienced for people who cherish these mythological elements, even if they don’t literally believe in them.
He later mentions the backlash over J. K. Rowling’s handling of Native American mythology in her “History of Magic in North America.” I’ve shared some of my thinking on that matter elsewhere, and much of what I say there applies here as well. My fiction is set in the United States of America. To write authentically, I feel I have an obligation to deal with the entire melting pot of cultures that I see around me on my way to work every morning. Therefore, this is what I wrote:
I’m not going to say Native American beliefs and folklore concerning magic, fantastic beasts, and so forth are off limits for fantasy writers. Nor, for that matter, should be the mythology of West Africans brought to North America as slaves. To be honest, leaving these elements out strikes me as more colonialistic than including them. Writing off black, Native American, or other non-white contributions to American life and culture leaves a story at best only half-told.
The challenge, especially for someone of European descent (something Ms. Rowling and I have in common), is to listen to these other cultures and go the second mile in attempting to depict them with dignity and integrity.
Of course, it’s up to you, my readers, to decide whether I’ve met that challenge.
Sometimes called the “Celtic Tree Alphabet,” Ogham (pronounced, roughly, Oh-um) is made up of straight lines intersecting a staff at various angles. Atlas Obscura has posted an informative article about 
Into the Wonder 